1. Field of the Invention
The present invention relates to the linearization of non linear networks and in particular concerns a fully adaptive linearizing circuit not using predistortion systems, particularly for microwave radiofrequency systems, and which is independent of the type of modulation. The present invention also concerns a method for linearizing the output of an apparatus, for instance a power amplifier.
2. Description of the Prior Art
The nonlinearity phenomena affect many fields of the electronic engineering. An important aspect, which is substantially the subject-matter of the present invention, is the one related to the nonlinearity of the amplifiers used in the electronic systems applied to telecommunication and in particular, others being not excluded, to digital signal transmission.
Present radio transmission systems, using complex modulations such as the multilevel ones, require more and more complex transceivers. In particular, in current digital signal transmissions, signals with high peak to average power ratios must be transmitted. In order to obtain a good quality of transmission by using these types of signals, it is necessary to keep the distortion value of the processed signal below a prefixed level.
This fact often forces the designers to use, for instance in transmission systems, class-A power amplifiers with high Back-Off values, hence with a low energy efficiency.
The main known linearizing techniques can be classified into four main different groups or methods: a) Feed-forward; b) Feed-back; c) Baseband numeric predistortion; and d) IF or RF analog predistortion. Each specific solution, excluding the Feed-back one which is adaptive by its very nature, can in turn be implemented according to an adaptive or non-adaptive technique.
In the adaptive technique, the linearizing apparatus observes the output produced by the power amplifier and, in dependence on the detected values, updates its internal parameters, so it minimizes the magnitude of the produced distortions. It is easy to understand how this is the ideal solution from the conceptual viewpoint, because it adapts through a suitable algorithm, the linearizing function to the actual behaviour of the system; in practice, the realization is unfortunately complex and, above all, expensive.
In the non-adaptive technique, on the contrary, the linearizing device is adjusted once and for all during the calibration, so as to assure the most satisfactory possible mean behaviour. The system is simpler, but with lower performances, since there is not feed back for controlling the actual status of the distortions coming out of the transmitter.
U.S. Pat. No. 5,742,201 discloses a predistortion linearizing device for an RF or microwave power amplifier. The circuit couples the input of an amplitude and phase controller through a feedback signal path to receive a portion of the amplified output signal. A second input receives a delayed portion of the input over a second signal path. The gain and phase regulations performed at the input to be amplified are controlled by a controller. However such known device has the following main feedbacks:
the system does not act instantaneously but with a delay due to outside controllers, and PA1 the feedback loop is based upon the estimation of two or more signal parameters and not upon a sole signal.